Studies are continuing to define the fimbrial adhesins of Actinomyces viscosus, an oral bacterium that colonizes tooth surfaces, forms dental plaque and contributes to the initiation of gingivitis and periodontal diseases. The type 1 fimbriae of Actinomyces mediate bacterial adsorption to saliva treated hydroxyapatite, an in vitro model of the tooth surface. The host receptors recognized in this interaction appear to be the acidic proline rich proteins found in saliva and as constituents of the acquired salivary pellicle. The structural gene for a 65 kilodalton type 1 fimbrial subunit has been localized on a 1.9 kilobase fragment of Actinomyces DNA using recombinant DNA technology. The encoded protein reacts with several monoclonal antibodies against type 1 fimbriae and has been purified using an affinity column prepared with one of these. Rabbit antibody against the cloned fimbrial subunit also reacts with Actinomyces type 1 fimbriae. Type 2 fimbriae account for the cell-associated lectin activity detected by the lactose sensitive coaggregation of Actinomyces with strains of Streptococcus sanguis. Interactions of this type appear to be involved in interbacterial adherence and the formation of specific microbial communities within dental plaque. The gene for a type 2 fimbrial subunit has been localized on a 2.5 kilobase fragment of Actinomyces DNA. It encodes a 59 kilodalton protein which has been purified and reacts with several specific monoclonal antibodies directed against type 2 fimbriae. The involvement of the cloned type 1 and type 2 fimbrial subunits in receptor binding by each type of fimbriae is being examined. These studies have begun to provide a structural basis for understanding the mechanisms of oral microbial adherence.